In recent years, electronic devices, as represented by a portable telephone or a personal computer, have been requested to have higher performance, and smaller size and thickness. With this request, the performance and density of electronic components, such as a CPU and an IC, are increased, which considerably increases amount of heat generation by the electronic components. Thus, suppressing a temperature rise in the electronic devices is an important problem to be addressed.
As a measure against heat, the following methods are used. For example, the heat generated in an electronic component, i.e. a heat generating body, is transmitted to a radiator, e.g. a heat sink, via a graphite sheet. Alternatively, the temperature of a heat spot is reduced by promptly diffusing the heat of the heat generating body via a graphite sheet. For such a purpose, a graphite sheet having large thermal conductivity, small thickness, and light weight is requested.
As such a graphite sheet, a pyrolytic graphite sheet and an expanded graphite sheet are used. The pyrolytic graphite sheet is a sheet made by pyrolyzing a polymer film, such as a polyimide film, at a high temperature. The expanded graphite sheet is a sheet made by heating and pressing acid-treated graphite powder into a film shape. In order to enhance the cooling capability with an increase in amount of heat generation by a heat generating body, it is necessary to increase the heat transport amount of the graphite sheet.
The heat transport amount of a graphite sheet is proportional to thermal conductance. When heat conducts in the lengthwise direction of the graphite sheet, the following relation holds.Thermal conductance (W/K)=thermal conductivity (W/mK)×cross sectional area of sheet÷length of sheetThus, when the principal surface of a sheet has a fixed shape, increasing the thermal conductivity and thickness of the sheet can increase the thermal conductance.
FIG. 8 is a perspective view of a conventional graphite complex. This complex is formed of pyrolytic graphite sheets 32A and 32B, and adhesive agent 31 bonding these graphite sheets.
When a pyrolytic graphite sheet is used as a graphite sheet for heat transportation and the thickness is increased so as to increase the thermal conductance, it is necessary to increase the thickness of the polymer films as the raw material. However, gas generated from the inside of the films at pyrolysis makes the sheet brittle in a powder form, and thus the thickness of the sheet cannot be increased. Thus, it is a common practice to laminate a plurality of pyrolytic graphite sheets 32A and 32B via adhesive agent 31, such as epoxy resin and urethane resin, so that a graphite complex is formed and the thermal conductance is increased (see Patent Literature 1, for example). However, in this structure, the extremely low thermal conductivity of adhesive agent 31 hinders the sufficient increase in the thermal conductance.
On the other hand, the expanded graphite sheet has a thermal conductivity in the direction of the principal surface lower than that of a pyrolytic graphite sheet. That is, when an expanded graphite sheet having a thickness equal to that of the pyrolytic graphite sheet is used as a graphite sheet for heat transportation, the expanded graphite sheet has a smaller thermal conductance. Further, the expanded graphite sheet is likely to delaminate as the thickness is increased, and thus is difficult to handle. Therefore, the thermal conductance of the expanded graphite sheet cannot be increased. For this reason, in order to increase the thickness of the expanded graphite sheet, it is a common practice to place a metal mesh body on the front and back sides or in the intermediate part of the expanded graphite sheet so that a graphite complex is formed (see Patent Literature 2, for example). However, in this structure, the thermal conductivity of the metal is smaller than that of the expanded graphite sheet, and thus the thermal conductance cannot be increased sufficiently.
Patent literature 1: Japanese Patent Unexamined Publication No. 2001-144237
Patent literature 2: Japanese Patent Unexamined Publication No. 2005-229100